Cable television providers receive multiple streams of compressed program data and then multiplex them into one of many outgoing QAM signals for distribution to users. In some cable television headends, for example, a single modulator may take as many as 8 or more incoming programs and combine them into a single outgoing RF QAM signal RF channel. In newer modulators many more incoming programs can be combined into multiple outgoing RF QAM signals on multiple channels. In the home, by selecting a particular program for viewing, the user effectively selects a desired RF channel on a Set Top Box (STB), which demodulates the received QAM signal on the selected channel to recover one of the compressed programs, and then decodes the selected program that the user wishes to view/hear.
It is desirous for a signal distribution company, such as the cable television company, to know various transmission parameters from reception through final delivery to the user. These parameters are useful, for example, to determine a source of signal degradation. For example, if cable subscribers receive a poor signal, they may contact the cable company to complain. In response, the cable company may measure or check performance metrics of the signals and the equipment along the various stages within the cable company to determine the source of the problem.
Signal to noise (S/N) ratios are an important metric to accurately characterize signal quality for digital QAM reception. IQ quadrature modulation is a method of modulating a carrier wave with two base-band input signals to produce a QAM signal. The two signals are oftentimes referred to as I (in-channel) and Q (quadrature-phase) components. IQ modulators are well known in the field of RF and microwave communications, finding use in both analog and digital modulation formats.
S/N ratios of a channel carrying an input QAM signal may be directly measured by using a spectrum analyzer. Spectrum analyzers may be relatively expensive, however, or otherwise unavailable to a cable television headend. Additionally, a spectrum analyzer requires a trained operator, who may not be available when the measurement is needed.
Although a Modulation Error Ratio (MER), described below, of QAM signals may be relatively easy to calculate from a received QAM signal's I and Q components, and although the MER may be related to the S/N of the channel, there is no known way to determine, from the demodulated I and Q components alone, the S/N of a channel carrying a QAM signal, the channel thus requiring measurement of the S/N of the channel before demodulation.
Embodiments of the invention address these and other limitations of the prior art.